The new generation mobile communication system features simplified network architecture, very low transmission/processing delay, high quality communications, high data rates and the like. Multi-carrier modulation (MCM) is regarded as one of key techniques in contemporary and future mobile communication systems. Orthogonal frequency division multiplexing (OFDM), a typical MCM scheme, has been widely used in fields of audio/video broadcasting and commercial communication systems, such as Digital Video Broadcasting (DVB) and Digital Audio Broadcasting (DAB) in Europe, Very-high-bit-rate Digital Subscriber Loop (VDSL), Power Line Communication (PLC), IEEE802.11a/g Wireless Local Area (WLAN), IEEE802.22 Wireless Regional Area Network (WRAN), IEEE802.16 World Interoperability for Microwave Access (WiMAX), and 3GPP Long Term Evolution (LTE) for Evolved Universal Terrestrial Radio Access (E-UTRA) systems, and the like.
The OFDM technique transforms a broadband channel into several parallel narrowband sub-channels, thus a high rate data stream transmitted in a frequency-selective fading channel is transformed into several low rate data streams transmitted on several parallel flat-fading sub-carriers. This can greatly enhance the system's capability in combating with the interference brought by the multi-path propagation channel. Further, Cyclic Prefix (CP) is adopted to transform linear convolution operations into circular convolution operations. According to characteristics of the circular convolution, when the length of CP is greater than the maximum channel delay spread, inter-symbol interference (ISI)-free channel equalization can be attained by using simple frequency domain zero forcing technique. The complexity of channel equalization is therefore remarkably reduced. Further, compared with conventional frequency division multiplexing (FDM) multi-carrier systems, OFDM systems allow overlapped sub-carriers as long as an orthogonality condition is satisfied, need no guard band, and require less demodulators on the receiver side. Thus, OFDM has advantages such as high spectral efficiency, simple receivers and low-cost hardware, and so on. Moreover, OFDM systems are very flexible in designing multiple access (MA) schemes and performing power allocation because the sub-carriers are independent to each other.
Conventional CP-OFDM scheme, however, has disadvantages in the following aspects.
(1) The usage of CP reduces the system power efficiency. Although the CP can effectively counteract the ISI, it consumes a portion of transmitting power and thus reduces the power efficiency of the transmitter. When the delay spread in a channel is greater than the length of the CP, there is still serious ISI.
(2) The system is very sensitive to carrier frequency offset (CFO).
(3) The relatively high peak-to-average power ratio (PAPR) places high requirements for the dynamic range of RF power amplifiers at the transmitter.
(4) The system has significant out of band emission. In order to solve the problem, sub-carriers close to the spectrum boundaries are often nulled, and this leads to reduced spectral efficiency and data transmission efficiency of the system.
The above disadvantages are mainly resulted from inherent characteristics of the OFDM system. Although methods can be taken to mitigate the adverse effects of the disadvantages, this increases the complexity in system design, and is not able to completely solve the problems. In this context, several other new MCM techniques have drawn close attention and are widely studied from multiple perspectives such as introducing filters having excellent time frequency localization (TFL) characteristics, increasing the distance between time-frequency grid points, changing the orthogonality condition, and so on. A typical system that has the above characteristics is the filter bank multi-carrier/offset quadrature amplitude modulation (FBMC/OQAM) system, which is also referred to as the OFDM/OQAM system.
The FBMC/OQAM system separately transmits a real part and an imaginary part of a complex symbol obtained from QAM mappings. The real part and the imaginary part of a complex symbol have an offset of half an FBMC symbol interval in between. The FBMC/OQAM system does not satisfy the orthogonality condition in the complex field, thus strictly speaking, it belongs to the category of non-orthogonal systems. However, the FBMC/OQAM system strictly satisfies the real-field orthogonality condition. As such, the FBMC/OQAM system can be regarded as a quasi-orthogonal system between an orthogonal system and a non-orthogonal system. Thus, relaxing the orthogonality condition to the real fields permits the use of filter banks with excellent TFL characteristics to trade-off between orthogonality and interference mitigation capabilities, and enables the FBMC/OQAM system to have good anti-interference ability. In general, the FBMC/OQAM technique not only maintains the orthogonality which simplifies the system design, but also adopts filter banks with excellent TFL characteristics to enhance the anti-interference ability of the system. These advantages make the FBMC/OQAM technique one of main candidate techniques for the 5th Generation Mobile Communications System.
Specifically, the FBMC/OQAM system has the following advantages.
(1) High spectral efficiency and power efficiency. Compared with CP-OFDM systems, in one OFDM symbol period, the FBMC/OQAM system transmits two real symbols (equivalent to one complex symbol). When CP is not taken into consideration, the FBMC/OQAM system has the same spectral efficiency and power efficiency with conventional OFDM systems. When CP is taken into consideration, the FBMC/OQAM system has higher spectral efficiency and effective transmitting power than CP-OFDM systems.
(2) Possession of both the anti ISI ability and the anti inter-carrier interference (ICI) ability. In the FBMC/OQAM system, relaxing the orthogonality condition from the complex field to the real fields permits the use of filter banks. Filter banks with excellent performances can provide good localization in both time and frequency, thus enable the FBMC/OQAM system to have both the anti ISI ability and the anti ICI ability without using CP.
(3) Low implementation complexity. The receiver of FBMC/OQAM system has lower complexity than non-orthogonal systems because it is still an orthogonal system. Furthermore, the FBMC/OQAM system can be implemented using Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT) to further reduce the design and implementation complexity of the system.
There are also some challenges in employing the FBMC/OQAM technique.
(1) Channel estimation is relatively difficult. The FBMC/OQAM system strictly satisfies the orthogonality condition only in the real fields, inherent interference exists between base functions of filter banks. The interference is imaginary-valued interference and can be eliminated by taking real parts of the received signal after channel equalization. But in the context of complex channel, the estimated channel is a superposition value of the inherent interference and the actual channel. When the data information around the pilot point is unknown, it is hard to eliminate the inherent interference which seriously affects the channel estimation accuracy.
(2) The system has high PAPR. The high PAPR is a result of the superposition of multiple signals, thus is an intrinsic characteristic of multi-carrier systems. As an MCM technique, FBMC/OQAM signal also has high PAPR.
(3) The processing procedures at the receiver side regarding equalization and demodulation are relatively complex. Received signal includes a complex interference component which cannot be completely eliminated by simply taking real parts. Residual interference is usually eliminated by interference estimation and cancellation, which would increase design complexity of the system. Moreover, since real parts and imaginary parts of complex data are transmitted separately, and then processed and synthesized at the receiver side. This procedure also increases the implementation complexity of the system. In addition, existence of the inherent interference makes it rather complex to integrate the FBMC/OQAM system with other systems such as multi-input multi-output (MIMO).